Exhaust gases from power plant boilers, melting furnaces, chemical factories and the like almost always contain corrosive substances which require special procedures in the design and operation of the plants if corrosion is to be avoided.
For example, the utilization of the heat content of flue gases from power plant boilers is limited by the fact that the temperature must not be lower than the dewpoint temperature in order to avoid corrosion.
That problem has been solved in a relatively simple manner by insuring that the temperature will not drop below a certain point and by selecting an exhaust gas temperature sufficiently above the dewpoint to prevent its lowering below the critical temperature during processing.
Nevertheless, active measures are often required to protect the walls of such apparatus against corrosion in most exhaust gas or flue gas cleaning processes.
This is especially important in the so called wet gas cleaning process regardless of the temperatures employed, because the corrosive substances dissolved in the liquid treating agent tend to attack the structural steel walls of the ducts and vessels. The corrosive substances may derive from the flue gas and are in their most virulent states in solution in scrubbing liquids.
Corrosion is also a problem in the so-called semidry and dry gas cleaning processes. In such systems it is not possible, as a rule, to avoid the corrosion problems simply by the selection of a temperature which is relatively harmless with respect to corrosion.
In semidry processes, the treating agents are introduced in a liquid form into the gas stream and are supposed to entirely evaporate so that only dry reaction products will be obtained.
Obviously, in that case, there is a risk of corrosion in the regions in which the gas stream still contains liquid treating agents or downstream of such regions where the temperature may decrease below the dewpoint if the supply of liquid is not properly controlled or if the flue gases are inadvertently under cooled.
Corrosion problems can also arise in flue gas cleaning using dry systems if the flue gases are cooled below the dewpoint because of economy or some other reasons having to do with process technology.
Faced with the problem that corrosion cannot normally be avoided by selection and maintenance of the process conditions, it is a common practice to provide liners for the structural steel walls to protect them against corrosion when the walls form parts of ducts and vessels in flue gas cleaning apparatus.
For example, the surfaces which are susceptible to corrosion may be coated with rubber or with glass fiber mats and polyester resins.
For both techniques, the substrate, i.e. the structural steel wall, must be very carefully pretreated to insure a complete bond between the coating and the substrate.
As a consequence, application of the corrosion resistant liner is a highly expensive procedure.
Furthermore, both methods or liner types have the serious drawback that the materials which are employed are combustible so that it is not possible to effect subsequent welding operations on parts of the wall sections. In practice, moreover, it has been found that it is not possible, utilizing prior art materials, to obtain a useful life of more than four to five years. That is by no means sufficient for most applications.
It has also been proposed to secure sheets of corrosion resistant steel by the so-called plug welding process to walls consisting of conventional structural steel.
In that technique, holes spaced apart at the desired fixing points are formed in the walls and the sheets are welded to the structural steel of the wall along the periphery of the holes. In other words, deposit welds are formed within the holes between the flanks of the hole and the lining sheet.
That process is relatively expensive and also is frequently rejected because the welding results in a change in composition of the corrosion resistance material of the sheet, e.g. a mixture of corrosion resistant materials and corrodible materials which can render the sheet at the weld point susceptible to corrosion as well.
Indeed, in practice it is observed that a new alloy which does not resist corrosion, is formed adjacent the deposit weld.
As the extent to which the sheet of corrosion resistant steel is melted will depend upon the skill and care of the welder, uniform results do not always occur. It is possible to burn through the sheets and because of compositional changes resulting from welding, which are largely uncontrollable, there is always the danger that the corrosion protection will be limited or no protection will be gained at all in regions adjacent to the weld deposit.
Another process avoids these drawbacks by providing a grid of strips of corrosion resistant steel to the wall and welding sheets of corrosion resistant steel subsequently to that grid. The grid spacing cannot exceed the spacing of the fixing seams and this spacing will depend on the thickness of the sheets of corrosion resistant steel. If the seam spacing is about 0.5 meter, the sheets of corrosion resistance steel must not be larger than 0.25 meter.sup.2 and must be secured by seam welds of a length of 2 meters. As a result, a complex and expensive welding process is required, both to secure the sheets to the grid and to secure the grid to the wall and, generally speaking, this more expensive process will not be adopted if another procedure is available.